Training Low: The Why & How of Low Carb Long Runs

Training Low: The How & Why of Low Carb Long Runs

One of the biggest questions during marathon training is: how do I avoid hitting the wall?

Pacing is important (no, you are not going to run a half marathon PR in the middle of a marathon) but nutrition makes a significant difference between a strong, successful marathon and a slow death march over the last 10K.

You spend months training your body to run a marathon or a half marathon, and the same principle should applying to your race day fueling strategy. Don’t just wait until race day and take a few gels randomly throughout the race.

Instead, you want to spend your training cycle not just training for the physiological demands of a marathon or half marathon, but also training for the metabolic demands of the race.

Training Low: The How & Why of Low Carb Long Runs

Low Carb Long Runs: The Why

One of the best ways to improve your marathon and half marathon times and avoid the wall is to improve your body’s fat-burning capacity, which is the ability to utilize fat as well as carbs in energy production.

The best way to improve your fat-burning capacity is to train your body to run without the ergogenic use of carbohydrates on sustained aerobic efforts. When your body runs low on carbohydrate, it’s forced to burn fat. By burning fat repeatedly on long runs, your body learns how to use fat, in addition to glycogen, as an energy source during long distance running. 

Note that increasing your fat-burning capacity still means you will have to fuel during the race and eat carbohydrates in your diet. You can’t completely shut off your carb-burning, nor should you. In fact, the low carb runs will also improves your body’s ability to store glycogen and tape into those glycogen reserves during a run.

Training to improve your fat-burning capacity doesn’t mean doing every run as a glycogen depletion run. Science and anecdotal evidence from runners and coaches support both training with and without carbohydrates on your long runs, so both should be included in your training.

In order to reap the benefits of both training with carbohydrates and increasing your fat-burning capacity, you should add both to your training by including low carb long runs for approximately half of your long runs within the 1:30-2:30 hour mark. Long runs without any mid-run carbohydrates can also help you train to avoid bonking during the race and hitting the dreaded wall.

Oftentimes I have the moderate, happy-medium approach, and training low carb on certain long runs certainly falls within that philosophy. It’s not as extreme as fasted, glycogen depletion runs (no food before or during the run), but it prepares your mind and body better for the nutritional and physiological demands of the marathon and half marathon than simply taking a gel every 45 minutes on every run over 90 minutes.

Matt Fitzgerald explains in his book The New Rules of Marathon and Half Marathon Nutrition, “As long as some of your harder workouts are carb-fueled and others are not, your fat-burning capacity will increase more than if you fueled every hard workout with carbs and your fitness will increase more than if you never took advantage of the performance-enhancing power of carbohydrate.”

Low carb long runs also prevent you from developing a crutch on gels and chews. Whether that crutch is mental or physical, it increases your chances of hitting the wall during your next race.

Low Carb Long Runs: The How

Eat your normal pre-long run snack/meal beforehand.

The purpose of these runs is not full glycogen depletion, so you want to have something in your tank. Low carb long runs are intended to simulate the final miles of your marathon or half marathon when your body may be running low on glycogen. You also don’t want to completely bonk the run, therefore starting on an empty tank is not ideal.

The key to these low carb long runs is not taking in any carbohydrate during the run itself. Drink water to thirst and take electrolytes as needed, but leave the gels and chews at home.

(Actually, I do keep an emergency gel on hand just in case – I’d rather fuel rather bonk and not finish my long run – but if you know that you will eat your fuel if you have it on hand, leave it at home).

Run without carbs only on easy long runs.

The faster you run, the more quickly you burn through glycogen (stored carbohydrates). At an easy running pace, your body utilizes both carbohydrates and fat as fuel. Running at paces above your aerobic threshold burn more carbs and less fat. Doing your low carb long runs at a faster pace thus doubly defeats the purpose: you’re more likely to bonk and you won’t be training in an aerobic zone that burns fat.

That said, you should be doing at least 50% of your marathon training long runs, if not more, at an easy pace. Otherwise, you’re likely to skyrocket your cortisol levels (too much stress on the body), overtrain, injure yourself, or at the very least leave your race in your training.

Refuel with carbohydrates and protein within 1 hour of completing your run.

After a low carb long run, your glycogen stores are primed to stock up on more carbohydrates. In fact, when your glycogen stores are repeatedly lowered and then refilled over time, you will actually be able to store more glycogen – which is ideal for endurance athletes.

These runs are not meant for weight loss, so don’t skimp on calories before or especially after the run. Really, you shouldn’t be actively trying to lose weight by means of restricting calories during marathon training.

Not eating a meal full of good carbohydrates and protein afterward will impair immune function, hinder post-run recovery, and overall decrease your athletic performance. All the benefits of a low carb training run will be lost, and you will probably actually increase your chances of bonking on race day if you consistently miss this prime refueling window.

Don’t skip carbs if you have hypoglycemia, diabetes, or other particular medical issues.

Ultimately, you must listen to your body. Everyone’s metabolism is different, so just because this approach works for me, another blogger, or your training partner, does not mean it will for you. Experiment with different fueling strategies, but don’t force something that does not work for you. 

Don’t go low-carb on all of your training runs.

You also need to teach your body how to digest and metabolize carbs on the run. Without practicing, you’ll end up with GI distress or, at the very least, your body is not fully utilizing the carbs you are giving it.

You also want to include some faster long runs in your marathon and half marathon training (ideally surges or segments at/near goal race pace). As I mentioned above, running faster burns through carbohydrates more quickly, so you want to consume the appropriate level of carbohydrates during quality workout long runs.

As this article from Runner’s Connect explains, ”although training with low glycogen during a hard workout results in enhanced muscle adaptations, these athletes had a lower power output during that workout compared to athletes with high carbohydrate availability. In other words, they weren’t able to do as much work when carbohydrate wasn’t available.”

So take in fuel on those harder long runs! Doing so will also train your stomach to digest fuel at marathon pace and help you figure out your marathon nutrition strategy.

Periodize low carb long runs.

The runs in which you withhold mid-run carbohydrates should occur during the early weeks of marathon training. During your sharpening phase (the last 3-5 long runs before the race), you do not want to withhold carbs on your runs. Instead, you want to ensure that you complete these long runs as best you can and you want to avoid depleting your glycogen stores so soon before the race.

Final Notes

Fat-burning is only one aspect of marathon nutrition and, for some people, it may not even make an impact on performance (see this study in the Scandinavian Journal of Medicine and Science in Sports). You still need to training your body to digest carbs while running, figure out which sports nutrition products work best for you, and eat a balanced diet of complex carbohydrates, lean protein, and healthy fats.

Want to learn more about how to create your own individualized fueling and hydration strategy for training and racing? I offer an e-course specifically designed to help you do so. Master Your Fueling and Hydration for Runners includes lessons, worksheets, formulas, and individualized feedback – learn more and sign up here

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18 Responses

  1. I like the approach of training with less carbs but also using more for some runs and less for others. I have done a few glycogen depleted long runs. I usually find that I can run without fuel pretty well, so it was really a matter of figuring out a race day plan that would work. I am constantly adjusting my fueling because I find that certain gels might work for one training cycle but not for another.

    1. Figuring out the fueling plan itself is hard – which is why I think a 50/50 split like you mentioned works so well. It’s so weird how what works changes from cycle to cycle! It’s like a constant science experiement.

  2. Have a bonked during a long run, half marathon or marathon. Yes, yes, yes. It always seems like once I nail down my nutrition, a bonk appears. It happens because every run is different. Most of the time it’s because of the weather or I slipped up on eating something. But when you are consistently aware of what works, it works. And it works well! Excellent info!

    1. Thank you! Oh the weather for sure throws fueling for a loop – even just the slightest increase in temperature or humidity! Bonking keeps us aware of what doesn’t work, at least.

  3. I train low, race high (ish). Especially now that I know what my body likes and I don’t have to test out things any more. I fuel just a bit during long long runs, and I generally don’t fuel for runs under 16 miles. I might now, simply because they will take me longer with my current paces, but I’ve found that it works for me generally.

  4. I do my weekday training runs (no more than 50 mins) on an empty stomach, mainly because I don’t have time to eat first! But I need a carby breakfast before my long run. And I run SO much better when I do! I usually don’t take gels until I start running more than 2:30 hours though. I think it’s all about funding what works best for you!

  5. This was such an informative post! I’m thinking a lot about fueling during my first marathon training cycle. I’m no longer using Clif Shotbloks and moving over to Honey Stinger chews for my long runs, testing it out for the first time on an 11-miler this weekend! I know I have done some runs on either low carbs or completely fasted in the spring, and I did end up having pretty good races so I wonder if it worked for me? In any event, this is such great food for thought (no pun intended)!

    1. Thank you! Good luck as you test out your fueling – it’s both exciting and a bit nerve-wracking to figure out that aspect of a first (or honestly, any) marathon. It sounds like it did work for you!

  6. Oh MAN this post is PERFECT timing! I was just crying a river YESTERDAY about how I’m trying to figure out my marathon fueling! I do fine during training runs, but race day, miles 15-20 ish are usually ROUGH! And I KNOW it’s my fueling – blood pressure dropping, feeling hungry, lightheaded, needing to walk, etc…
    Anyhow, this post kinda validated something for me…last summer, I trained with the 11:30 pace group for my marathon, and I ended up running that 11:30 pace for the race…did the same thing this past winter – training with the 11:00 pace group, ran Pittsburgh at a 11:00 pace…NOW I’m TRYING to move around the pace groups and run some of the longer runs at a faster pace because I’ve felt like I just needed to TRAIN some at the faster pace so I can practice/prepare how I actually end up feeling on race day…is that logical? Your post made it seem logical, but maybe I’m still missing the point, haha.
    I’m JUST about to get into the 16-20 mile long runs starting NEXT weekend, so maybe this weekend, for the 14 miler, I’ll attempt a faster pace group with a low carb fuel strategy?

    1. Hi Nikki, I would recommend either running with a faster pace group OR not taking in carbohydrates during the run – not both. As I mentioned in the post, you want to run low carb long runs at an easy, comfortable pace, which is often slower than your goal marathon pace. Otherwise, your body will burn through carbs too quickly, since the faster you run the more your use carbs as fuel. If you’re running with a faster pace group for your long run, you want to take in adequate carbohydrates during the run.
      If you’re interesting in figuring out marathon fueling, my Master Your Fueling and Hydration e-course may be helpful for you! The course focuses heavily on marathon fueling and I provide feedback on your fueling plan.

  7. Fantastic and informative post! I have been amazed at how much my body has adapted during my 2nd training cycle. I’ve been experimenting a lot more with what I eat before a long run – the other day I was able to run an 18 miler feeling fantastic on only 1 gel where in the past I would have 2 or more. I don’t know if that’s my body just becoming more efficient at running or a combination of what I ate pre-run and the night before?

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